PROLOG TURTLE GRAPHICS
        Contributed by Salleh Mustaffa, University of Manchester
                  Received on the 17th of November 1987
                   Shelved on the 6th of December 1987
                  Amended on the 17th Of December 1987


           Copyright (c) 1987 by David Lau-Kee, Univ. of York.
  Permission is granted to use or modify this code provided this notice
             is included in all complete or partial copies.


I enclose a turtle graphics program written by David Lau-Kee. It came
from a posting to the Prolog Digest some months ago. It had a few
syntactic errors which I have corrected. The program is said to be
terminal specific and probably works best on a colour VT-100 terminal,
but I've only been able to see it in action on a monochrome VT-100. I
have not been able to get it to work properly on a Sun terminal (even
after hours of flipping through the ANSII manuals for the Sun). I think
it is also Unix specific, but I may be wrong. I suspect the initial
errors that I had to contend with initially are due to the control
characters in the program which are not properly transmitted in the
mail. [Salleh Mustaffa]


The basic idea came from a colleague's thought that it would be useful
to be able to show the progress of instantiations in a goal. To be
really useful there needs to be some means of "unshowing" the partial
groundings, so that the display is always consistent with a snapshot of
the state of the system.

Really this program is only an experiment on that concept. It has *lots*
of rough edges and *lots* of nasty constructs... However, it does
provide an interesting way of "watching" Prolog at work.

A much reduced "turtle graphics" environment is presented. (Basically
you can tell the turtle to turn 90 deg. clockwise or anticlockwise, and
to move forward X steps.) The "interesting" feature is that backtracking
over these primitives results in them being undone.

For example, I have defined anysq(X) to be four sets of 'forward X
steps, turn'. If I try to satisfy 'anysq(4), fail.' then all the
possible solutions to:

'anysq(4).', where 'forward (drawing) X' and 'backward (erasing) X'
satisfy the goal 'forward X', and 'turn clockwise' and 'turn
anticlockwise' satisfy 'turn' are drawn.

I wrote the program for C-Prolog 1.5, but it wouldn't require much
effort to move it to other Prologs. It *is* terminal specific - I use a
Microcolour m2200 in ANSII mode (basically vt100 plus colour) - but
again, if your terminal has ANSII features, it won't be a problem to
hack it straight. One thing may be worth thinking about: When a line is
undrawn it is sometimes helpful to leave a record of it. I cope with
this by switching the undo colour from black to white. This is ok on a
colour terminal where the lines themselves are other than white, but it
would be a bit confusing in monochrome... maybe half-intensity or
something?

The program is slower than it really needs to be. Previously the undo
would act like paint stripper, cutting right down to the black of the
background. This version paints in layers, undoing strips off a layer.
(Great if you've got a lot of colours.) For a 5-fold speed increase take
out the layers. [David Lau-Kee]


I've reformatted the program, changed % comments to /* .. */, and made a
few other syntactic changes. [JNP]


SIZE : 18 kilobytes.


CHECKED ON EDINBURGH-COMPATIBLE (POPLOG) PROLOG : yes.

    Doesn't  use  any non-standard  predicates,  but  won't run  on  our
    (VT100) terminals. However,  the appearance of the  screen is almost
    correct -  small changes to the  terminal-handling predicates should
    make it work.


PORTABILITY :

    the cursor-moving commands are terminal-specific.


INTERNAL DOCUMENTATION :

    The cursor-addressing  predicates carry  very brief  descriptions of
    what they're supposed to do.